Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the extreme ultraviolet (EUV) range, e.g. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
Among others, the operation of mirrors under grazing incidence is known. Such mirrors operated under grazing incidence are understood here and in the following to mean mirrors for which the reflection angles, which occur during the reflection of the EUV radiation and relate to the respective surface normal, are at least 65°. Sometimes, such mirrors are also referred to in an abbreviated fashion as GI mirrors (“grazing incidence”). In principle, the use of such GI mirrors is desirable, inter alia in view of the comparatively high achievable reflectivities (of e.g. 80% and more).
In contrast to, for instance, mirrors operated under normal incidence (also referred to as NI mirrors; “normal incidence”), such GI mirrors do not require a multilayer system in the form of an alternating sequence of numerous individual layers made of at least two different layer materials as a reflection layer for obtaining the respective reflectivities; instead, they require only a single layer, which may, for example, consist of ruthenium (Ru) and which may, for example, have a typical layer thickness in the region of 40 nm.
However, even if GI mirrors are used, a problem occurring in practice, in particular in the projection lens, is that electromagnetic radiation, for example in the form of transmitted light or stray light, may reach through the respective reflection layer to the mirror substrate; for example, this is promoted by virtue of the relevant electromagnetic radiation differing from the actual used light in terms of the angle and/or wavelength distribution. The relevant electromagnetic radiation reaching the mirror substrate may then cause damage or changes, for example in the form of radiation-induced local density changes (compacting), in the mirror substrate material, which in turn leads to unwanted changes in the wavefront during the operation of the optical system and hence, ultimately, to an impairment of the performance of the optical system or the projection exposure apparatus.
In relation to the prior art, reference is made merely by way of example to DE 10 2009 032 779 A1, DE 10 2009 054 653 A1, US 2013/0038929 A1, DE 10 2009 049 640 A1 and DE 10 2012 202 675 A1.